The optical constants were obtained by measuring the amplitude ratio \(\Psi \)  and phase difference \(\Delta \)  of the p- and s-polarized light [1]} reflected by the samples at different incidence angles \(\theta = 55\) °, 60°, 65°, close to the coating Brewster angle. The optical response of the substrates (silicon wafers with a thin native oxide layer and unpolished rear surface in order to avoid backside spurious reflections) was characterized with prior dedicated measurements and analyses [2]}.
As in previous works on similar systems [2]}, [4]}, we analyzed the data through a three-layer model consisting of substrate, thin film and surface layer. The latter, accounting for surface roughness, was defined via a Bruggeman effective medium approximation. The resulting coating roughness was less than 1 nm, a value comparable with those measured with an atomic-force microscope on similar samples [4]}. Being less than 1% of the coating thickness, the surface layer did not affect substantially the data fit, especially in the near-infrared region.
